Timing of incision of the western margin of the Colorado Plateau, new thermochronological data from Zion Canyon

The Colorado Plateau is a typical continental orogenic plateau characterized by a low-relief surface at high elevation that has been incised by the Colorado River system, forming outstanding canyons including the Grand Canyon and Zion Canyon. Although canyons are key features of ecosystems and water resources across the Colorado Plateau and form some of the most dramatic landscapes on Earth, the chronology of plateau uplift, subsequent canyon incision, and the controlling processes remain debated. The relative importance of mantle processes, tectonics, pre-existing geological structures, river drainage evolution, and climate remains controversial. Most studies addressing the timing of canyon incision and landscape evolution across the Colorado Plateau have focused on the Grand Canyon which shows the most spectacular incision with more than 1500 m of relief. Two end-member models of the Grand Canyon incision have been proposed: a 80-60 Ma incision or a 6-5 Ma incision. These models have important implications for processes driving Colorado Plateau uplift and incision, and for feedbacks on regional climate. However, studies quantifying the timing of canyon incision and surface uplift are lacking in other areas of the plateau. We used apatite fission-track and (U-Th-Sm)/He analysis to infer the incision history of Zion Canyon by the Virgin River on the Western margin of the Colorado Plateau. These low temperature thermochronological systems are sensitive to temperature ranging from 120 to 50°C. Despite the canyon only being a maximum of ~1 km deep, a high local geothermal gradient of >50°C / km means these thermochronometers provide a record of the timing of this incision. Preliminary inverse thermal modelling of apatite fission-track and (U-Th-Sm)/He data suggest reheating following Jurassic deposition to maximum temperatures of ~70-80 °C during the later Cenozoic, with onset of incision-related increased cooling rates in the last 10 Ma. Our results are in agreement with the recent work of Walk et al. (2019) indicating incision by the Virgin River during the last 4 to 3 Myr in the Zion area. Together with existing structural cross-sections and reconstructions of the timing of surface uplift and incision by the Virgin River in the Zion area, our thermochronological data support that Zion Canyon was carved since the late Miocene following tectonically driven rock and surface uplift along the western edge of the Colorado Plateau.